1. Field of the Invention
This invention relates generally to transceiver architecture in a wireless portable communication device. More particularly, the invention relates to an internal calibration system for a radio frequency (RF) transmitter.
2. Related Art
Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. An RF transmitter must transmit using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. However, RF transmitters are imperfect electronic devices and suffer impairments from a variety of sources. For example, common transmitter impairments include gain imbalance, phase imbalance and direct current (DC) offset as it relates to the in-phase (I) and quadrature-phase (Q) components of a transmit signal.
These and other impairments are compensated today during manufacture of an RF transmitter and typically involve setting the transmitter to send a known signal, which can be, for example, a sinusoid. This known signal is examined by external test equipment and compared with the expected signal. The transmitter is adjusted using information extracted from the examined test signal. Unfortunately, this is a time consuming process that requires external test and measurement equipment.
Another prior solution to compensate for transmitter impairments monitors the output of a transmit voltage controlled oscillator (VCO) using receiver circuitry located in the same device as the transmit circuitry. Unfortunately, this technique requires significant area on the device to achieve the required amplification levels. This technique also only measures a small portion of the overall transmit chain by inferring the performance of the transmitter by analyzing the output of the VCO, thus ignoring impairments that may be added by other components in the transmit chain.
Another prior technique for analyzing and compensating transmitter impairments routes the transmitter output directly to receiver circuitry on the device so the receiver circuitry can observe the transmit signal. Unfortunately, in a time division multiple access (TDMA) communication system, such as the Global system for Mobile Communication (GSM), there is normally no full duplex transmit/receive capability, thus making this technique difficult and expensive to perform.
Therefore, it would be desirable to efficiently and quickly detect and compensate transmitter impairments in a communication device.